Today's transmission systems build basically on time division multiplexing. This is, a number of digital tributary signals are interleaved to form a multiplex transmission signal which is then transmitted over the transmission system. The tributary signals, however, do not necessarily derive their timing from the same reference timing source and may further be subject to varying delay. Therefore, in order to allow them to be interleaved, their bitrates need to be adjusted to a common rate. This is called synchronization. There are different techniques known for synchronization of which the most important one is bit or byte justification. Additional stuff bits or bytes are added to or omitted from the tributary signals if necessary to adapt their data rate.
One important transmission system is known as SDH (Synchronous Digital Hierarchy) which bases on a frame format and multiplexing hierarchy defined in ITU-T G.707 (2000). The basic transport frame in SDH is called STM-N (Synchronous Transport Module, N=1, 4, 16, 64, 256) and tributary signals are carried within this STM-N frame. In order to form the frame, tributary signals are first mapped into what is called a container (C-N, N=4, 3, 2, 12, or 11) by positive/zero/negative bit stuffing. This brings them up to a common rate. Together with some path overhead, they are called virtual containers VC-N. Lower order virtual containers (VC-3, 2, 12, 11) are multiplexed into a higher order virtual container (VC-4) which is then mapped into an STM-1 frame.
However, virtual containers coming from different places in a network may still have slightly different rates and phases. In order to compensate for this phase and frequency differences, pointers address the virtual containers inside their higher order structure. In particular a first pointer called AU pointer (AU: administrative unit) addresses the VC-4 inside the STM-1 and TU pointers (TU: tributary unit) address the lower order VCs inside the VC-4. Each pointer indicates in number of bytes the phase offset between the first possible byte position and the actual beginning of the VC. In order to compensate for frequency differences, stuff bytes (either three for the VC-4 or one for lower order VCs) may be inserted and the pointer is adjusted accordingly by +/−1.
This is called pointer justification event and is in principle the same as positive/zero/negative byte justification. Justification can happen on any level of VC.
Another recently developed transmission system is called OTN (Optical Transport Network) and is based on a frame structure and multiplexing hierarchy defined in ITU-T G.709 (2001). The multiplexing of tributary signals is likewise based on bit and byte justification.
However, at the far end of a transmission line or path, the tributary signal is to be extracted from the transport frame structure and the original data rate must be recovered. Recovery of the original data rate is a primary issue in any synchronous or plesiochronous transmission systems (as opposed to asynchronous or packet oriented transmission systems) since these types of transmission systems are required not to affect the data rate of a tributary signal.
Extraction and recovery of the tributary signal at its original data rate is basically performed by removing any stuff bits or bytes, writing the signal into a buffer memory and reading the tributary signal from the buffer at the rate of a local oscillator. In order to avoid over- or underflow of the buffer, the frequency of the local oscillator is controlled so as to run faster when the buffer is too full and conversely to run slower when the buffer is too empty. The device that performs this extraction and recovery is called a desynchronizer and the process is called desynchronization.
A basic desynchronizer which operates this way is known from R. Urbansky, “Simulation Results and Field Trial Experience of Justification Jitter”, 6th World Telecommunication Forum part 2, 10-15 Oct. 1991, Technical Symposium, Integration Interoperation and Interconnection: The Way to Global Services, Geneva, Switzerland, Int. Telecommun. Union, 1991, p. 45-49, vol. 3 of 3, which is incorporated by reference herein.
However, justification events occurring in the multiplex signals will cause the local oscillator to retard or accelerate, which may cause high and low frequency phase variations, called jitter and wander, respectively, in the recovered tributary signal. Another particular source for low frequency jitter and wander is the oscillator control process as such, which is based on measuring the filling level of the buffer or equally comparing read and write timing phases. As the data bytes of the tributary signal are not equally distributed over the multiplex signal, gaps occur in the clock of the tributary data stream which are due to overhead or FEC (forward error correction) bytes, respectively blocked from entering the buffer. These gaps affect the comparison process between read and write timing rate of the buffer and thus produce low frequency jitter and wander. Jitter and wander, however, are impairments to the original tributary signal and therefore, these need to be avoided or reduced to an acceptable minimum.
There are a number of proposals of how to implement a desynchronizer which reduces jitter and wander produced in the recovered tributary signal.
U.S. Pat. No. 6,188,685 describes a method which first restructures the multiplex signal so as to spread the overhead section equally along the frame and then extracts and desynchronizes the tributary signals. This avoids large gaps in the payload clock of the multiplex signal and thus makes phase comparison, which is the basis for controlling the local oscillator, more accurate.
U.S. Pat. No. 5,781,597 describes a desynchronizer which counts and compares frame bytes rather than only payload bytes. Justification events which occur in the multiplex signal are taken into account by an additional correction parameter called leak factor. This avoids the gaps in the payload write clock affecting the oscillator control process.
WO 93/12600 describes a desynchonizer of the above-described type, where the local clock generator is tuned in synchronism with the multiframe rate of the virtual container that contains the tributary signal to be recovered. U.S. Pat. No. 5,131,013 described a similar approach where the local clock generator is tuned in synchronism with the frame clock of the virtual container that contains the tributary signal to be extracted.
However, the known desynchronization techniques are either unsatisfactory or complex. It is therefore an object of the present invention to provide another desynchronizer and corresponding method for desynchronization which produces reduced jitter and wander with low technical effort.